The Spanish influenza pandemic of 1918-19 has been dubbed "the mother of all pandemics" by researchers at the Centers for Disease Control and Prevention (CDC) because it remains one of the most lethal outbreaks of infectious disease in human history. It is credited with up to 100 million deaths around the world.
According to the CDC, systemic symptoms of Spanish influenza, like seasonal influenza, include fever, fatigue, muscle and joint pain, headache and lack of appetite. The severity of these systemic symptoms is proportionate to the degree of immune activation.
In a 2006 study in the journal Nature, researchers using molecular genetics techniques demonstrated that the Spanish influenza triggered exceptionally high and sustained expression of pro-inflammatory cytokines and chemokines by the immune system, accounting for the relative severity of Spanish influenza symptoms compared to other flu viruses.
According to the CDC, respiratory symptoms of Spanish influenza include cough, sore throat, runny nose, nasal congestion and breathing problems. Although similar symptoms are seen with all influenza viruses, they are particularly severe with the Spanish influenza. According to the 2006 study in Nature, modern histopathological analysis of autopsy samples from Spanish flu cases demonstrated extraordinary damage to the lungs with acute focal inflammation of the airways (bronchi) and soft tissue (alveoli) of the lungs. These changes were associated with massive swelling, bleeding and near-complete destruction of the surface cell layer of the lungs. The first two findings would have resulted in markedly impaired oxygen diffusion, while the latter would have provided the ideal breeding ground for bacteria.
Pneumonia and Death
In a 2008 article published in the Journal of Infectious Disease, researchers from the National Institutes of Health reported their finding that secondary bacterial pneumonia was the predominant cause of death during the Spanish influenza pandemic. Secondary bacterial pneumonia is a well-known complication of all types of influenza--even today--and occurs when bacteria invade an airway significantly damaged by influenza infection. Symptoms of pneumonia include chills, shortness of breath and difficulty breathing. People with symptoms of pneumonia always require medical attention.